Spectroscopic detection of a phytochrome-like photoreceptor in the myxomycete Physarum polycephalum and the kinetic mechanism for the photocontrol ofsporulation by P-fr

Sporulation of the true slime mold Physarum polycephalum (Myxomycetales) ca n be triggered by the far-red/red reversible Physarum phytochrome. Physarum plasmodia were analyzed with a purpose-built dual-wavelength photometer th at is designed for phytochrome measurements. A photoreversible absorbance c hange at 670 nm was monitored after actinic red (R) and far-red (FR) irradi ation of starved plasmodia, confirming the occurrence of a phytochrome-like photoreceptor in Physarum spectroscopically. These signals were not found in growing plasmodia, suggesting the Physarum phytochrome to be synthesized during starvation, which makes the cells competent for the photoinduction of sporulation. The photoconversion rates by R and FR light were similar in the phytochromes of Physarum and etiolated oat shoots. In dark-grown Physa rum plasmodia that had not been preexposed to any light only R induced a de tectable absorbance change while FR did not. This indicates that most (at l east 90%) of the photoreversible pigment occurs in the red-absorbing form. Since the effectiveness of FR in triggering sporulation was enhanced by pre irradiation with R, it is concluded that at least part of the P-r can be ph otoconverted to the active P-fr photoreceptor species. We propose a kinetic mechanism for the photocontrol of sporulation by photoconversion of P-fr, which may also hold for the high-irradiance response to FR in Arabidopsis a nd Cuscuta.